Monday October 12 2009. Put the date in your diary. It marks the start of the pivotal week in which MPs return to Westminster after their absurdly long recess. Much more importantly, it signals the opening of the brief autumn window that will finally determine whether Gordon Brown leads Labour into the 2010 general election – with all that that implies for his party.

The default view at Westminster is that the leadership issue is dead and buried. Brown's opponents had their chance in the June reshuffle, the argument runs, but turned it into the night of the blunt knives. Brown may be a diminished figure, but he is still respected. His future is simply not being talked about in the way it was a few months ago or at this time last year, a minister stresses. The die is cast, he says. Gordon will lead.

Yet even this minister, no rebel, acknowledges that the mood could still change. Another Brown loyalist says the prime minister's position depends on the public, which, when you think about it, is hardly a ringing endorsement. A third loyalist says that if he thought Brown was dragging the party down, he would tell him he must do the decent thing and quit. One Labour official whom I texted yesterday for his current assessment of "the Gordon question" replied simply: "Aaaaaaarrrrgggghhhh!"

Others, though, are dedicated to forcing the issue. An active network of MPs and peers now exists, involving some names you might expect, but also others – including big ones – whose participation would surprise you. This group, like probably the majority of Labour MPs, accepts that Brown is a liability to his party's election prospects. Unlike the majority, though, they claim to think something can be done about it. They believe the window of opportunity, if it comes, will be in the two or three weeks after October 12. If Brown can be pushed, then this is the time. They say they are ready to try.

Their core case is familiar. Brown is a bad and failed leader. Indecisive. Cautious. Doesn't know what he believes in. Always calculating, often badly. Unable to inspire. The polls are worse than they have ever been and nothing Brown does seems able to change them. Labour now faces not just a conventional defeat but a historic wipeout. A new and better leader, elected after a proper contest and an overdue strategic debate, can turn a new page, would allow Labour to be heard, could re-energise the core vote and keep the losses to a minimum, enabling Labour to stay in the game.

Whatever one thinks of that case, the natural first reaction to this has to be sceptical. What has changed since the last time? Sure, the polls are terrible for Labour now, but they were terrible then. The Labour party seems endlessly willing to take byelection humiliations on the chin, so don't expect Glasgow North East to be a knockout. The field of potential successors to Brown, none of them the dream candidate, is also unchanged. If the October 2008 and June 2009 plots failed, why should October 2009 be any different? But the plotters have an answer to that – and it is one that smart loyalists also acknowledge. The answer is that time is now on their side.

If Brown had been toppled by the earlier challenges, this argument runs, there would have been an irresistible demand for an immediate general election. Fear of that outcome, fuelled by Labour's dismal electoral performances, was the main reason those moves lacked wider support among backbenchers. To topple Brown now, on the other hand, would be different, it is claimed. There would still be calls for an early election, but they would now have less purchase. There will be an election next spring anyway. No one wants a winter poll. So the public will have its say soon enough. And in the meantime, Labour can begin a long overdue grown-up conversation with the voters again.

There is a secondary timing issue here too. In June Lord Mandelson scared off some wavering rebels by warning that an early election might allow a Cameron government to call an early autumn referendum on the as yet unratified Lisbon treaty. A "no" there might wreck the EU and make Britain the pariah of Europe. By November, though, that argument may also have less potency. If the Irish vote yes in their October 2 referendum and the other states ratify too, then Lisbon will be a done deal before a Tory win. Cameron would not be able to reopen the treaty.

The plotters intend no move at the Labour conference at the end of this month. Even in its reduced condition, Labour still demands loyalty, especially in the run-up to a general election. The mood in and after Brighton, though, will be vital. It will matter whether Brown's speech is judged a success. And external events may count too. Will the Pittsburgh G20 summit on the eve of Labour conference show Brown again saving the world or, as some reports imply, increasingly isolated? How will Germany's September 27 election affect the mood?

When Westminster reassembles, everything will come down to three things. The first is Labour's collective state of mind. Today, after a week where Brown's enemies have pummelled him for his dismal equivocations over the Lockerbie bomber, the backbench verdict might be pretty close to YouGov's poll verdict in yesterday's Sun, with a mere 16% of voters thinking Brown is doing a good job. It would be "in the name of God, go" time. Yet five weeks is a long time in politics. The post-conference season polls will be key.

The second is whether, if there genuinely is continuing despair over Brown, the cabinet would act on it. They stood by him in June, remember. The role of senior ministers without leadership expectations, such as Mandelson, Alistair Darling and Jack Straw, will be pivotal here – Mandelson above all. In the end, though, the person who most matters is Brown himself. He might fight, or he might walk away. Good judges are divided. No one really knows, not even he.

Will any of this happen? History, both of Labour in general and of the plots against Brown in particular, suggests it will not. People who are often right take this view. But history, as Simon Jenkins wrote the other day, never repeats itself. Perhaps this time history will be turned on its head. The first October revolution brought one Great Leader to power. The next one may usher this Great Leader out of the door. Brown is not safe yet. Remember the date.